WO-A-01/98393 describes 2K (2-component) coating compositions comprising a polyol binder component and a polyisocyanate crosslinker component partly functionalized with alkoxysilyl groups. These coating compositions are used as primers and are optimized for adhesion to metallic substrates, especially aluminum substrates. Over these coating compositions, as part of an OEM finish or a refinish, it is possible to apply basecoat/clearcoat systems. In terms of scratch resistance and weathering stability, the coating compositions of WO 01/98393 are not optimized.
EP-A-0 994 117 describes moisture-curable mixtures comprising a polyol component and a polyisocyanate component which may partly have been reacted with a monoalkoxysilylalkylamine that has undergone reaction preferably to an aspartate. Although coatings formed from such mixtures do have a certain hardness, they are nevertheless of only limited suitability for OEM applications in terms of their weathering stability and, in particular, their scratch resistance.
US-A-2006/0217472 describes coating compositions which can comprise a hydroxy-functional acrylate, a low molecular mass polyol component, a polyisocyanate, and an amino-functional alkoxysilyl component, preferably bisalkoxysilylamine. Such coating compositions are used as clearcoat material in basecoat/clearcoat systems and lead to scratchproof coatings. Coating compositions of this kind, however, have only very limited storage qualities, and the resulting coatings have low weathering stability, particularly with respect to UV radiation in a wet/dry cycle.
WO 2006/042585 describes clearcoat materials which are suitable for OEM finishing and which as their main binder component comprise polyisocyanates whose isocyanate groups, preferably to an extent of more than 90 mol %, have undergone reaction with bisalkoxysilylamines. Clearcoat materials of this kind combine excellent scratch resistance with high chemical and weathering resistance. But there is still a need for a further improvement in the weathering stability, particularly with respect to cracking under UV irradiation in a wet/dry cycle, with retention of the high level of scratch proofing.
EP-A-1 273 640 describes 2K coating compositions composed of a polyol component and of a crosslinker component consisting of aliphatic and/or cycloaliphatic polyisocyanates, 0.1 to 95 mol % of the free isocyanate groups originally present having undergone reaction with bisalkoxysilylamine. These coating compositions can be used for OEM finishing and when fully cured combine good scratch resistance with effective resistance to environmental influences. Nevertheless, these coating compositions have a particularly strong propensity toward aftercrosslinking, with the consequence—directly after thermal curing to completion—of inadequate scratch resistance of the coatings. The significant after crosslinking likewise impacts adversely on the weathering stability, since there is an increased risk of stress cracks.
For the original finishing of automobiles it is possible to employ relatively high curing temperatures, since at that point the automobile body does not contain any temperature-sensitive parts. The situation with refinishing is different. When, for example, defects in the ready-painted auto require partial repair in line manufacture, or when, in the event of subsequent minor damage, only part of the surface is to be refinished, it is necessary to use coating materials which cure at low temperatures, in order not to jeopardize temperature-sensitive parts such as tires and parts made of plastic. Silanized systems are able to be cured at relatively low temperatures, but the isocyanate crosslinking is generally sufficient only at elevated temperatures.
In the case of partly silanized systems which are additionally crosslinked by polyisocyanates, with formation of urethane, it is common to use blocked phosphoric ester derivatives as catalysts. In these systems the acidic action of the phosphoric ester is neutralized by a basic component. Blocking is usually effected using amines, such as triethylamine for example. Above a certain temperature, the blocking is eliminated, the phosphorus compounds bring about acidic hydrolysis of the alkoxysilane compounds, and the triethylamine, on account of its low vapor pressure, departs the coating. In this case, generally speaking, there is no acidic catalysis of the isocyanate-polyol reaction.
It was an object of the present invention to provide coating compositions, particularly for the clearcoat film in OEM finishes and automotive refinishes, that lead to a network with a high degree of weathering stability, the unwanted formation of moieties unstable to hydrolysis and weathering being very largely suppressed, in order to ensure high acid resistance. In addition, the coating compositions ought to lead to coatings which have a high degree of scratchproofing directly after thermal curing, and in particular a high retention of gloss after scratch exposure. Moreover, the coatings and coating systems, especially the clearcoat systems, ought to be able to be produced even in film thicknesses >40 μm without stress cracks occurring. This is a key requirement for the use of the coatings and coating systems, particularly of the clearcoat systems, in the technologically and esthetically particularly demanding field of automotive OEM finishing.
The intention in particular was to provide clearcoat systems featuring high resistance, particularly to cracking, under weathering with UV radiation in a wet/dry cycle, in combination with outstanding scratchproofing.
Furthermore, the new coating compositions ought to be preparable easily and with very good reproducibility, and ought not to present any environmental problems during application of the coating material.
A key problem facing the present invention is to achieve complete crosslinking at low temperatures for coating materials which are cured by hydrolysis of alkoxysilane compounds and, additionally, by the reaction of isocyanate groups with hydroxyl groups. Surprisingly it has been found that this problem can be solved through the use of bicyclic amines for blocking phosphoric acid catalysts.